Flexible arms or, as they are often called, articulable columns, have many uses. For example, they are often used for positioning tools, article supports, or for locking measuring apparatus. In surgery, it is common practice to mount them as adjustable supporting brackets on a side rail of an operating table to support retractors, endoscopes and other surgical devices.
U.S. Pat. No. 4,949,927 discloses an articulable column and, more particularly, describes prior art columns of the ball and socket type which are flexible in their normal state and which, by application of tension from a central cable, become rigid.
Recent developments in heart surgery require stronger and more rigid adjustable brackets. In particular, a procedure has been introduced for carrying out cardiac bypass surgery without stopping the patient's heart. In this procedure, a device called a “tissue stabilizer” is used.
A specific prior art example, U.S. Pat. No. 5,727,569 teaches that the tissue stabilizer is attached to the wall of the heart by drawing a vacuum in an array of suction cups. With one or more such devices attached to the wall of the heart, the site at which the repair is to take place can be held fixed while the heart continues to beat.
A tissue stabilizer is often supported using a lockable articulating column, such as disclosed in U.S. Pat. No. 5,348,259. A lockable articulating column is described as a flexible, articulable column having a central tensioning cable strung through a series of ball and socket members. Each socket member has a conical opening with internal teeth engagable with a ball made of an elastomeric polymer. When the cable is tensioned, the sockets move toward each other and the balls become indented by the teeth of the socket. The column becomes rigid when the central cable is tensioned. Releasing the tension returns the column to the flexible state.
FIG. 1 is an elevational view illustrating a tissue stabilizer supported from the side rail of an operating table by a bracket as found in the prior art of U.S. Pat. No. 5,899,425.
The assembly in FIG. 1 includes vertical post 10 attached to side-rail 12 of an operating table (not shown) by a clamp 14. The post 10 often has plural facets, which cooperate with the clamp to prevent rotation of the post relative to the clamp. A tension block 16, mounted at the top of post 10, comprises a mounting block 18 and a rotatable member 20.
In FIG. 1, one end of a flexible arm 24 is connected to the side of mounting block 18 opposite to the side having the rotatable member 20. Flexible arm 24 comprises a series of articulating elements connected to one another by ball-and-socket joints. The number of ball and socket members may be increased or decreased depending on the use of the articulating column. The flexible arm 24 has a clamp assembly 26 mounted at its other end. The clamp assembly 26 holds the shank 28 of tissue stabilizer 30.
Typically, tensioned mounting block 18 has an internal passage receiving a screw 32. Affixed to the screw is a transverse pin riding in slots formed in opposite sides of mounting block 18. The engagement of the pin with the slots prevents the screw from rotating relative to mounting block 18. The threads of the screw engage internal threads in a rotatable member 20, which also has an internal shoulder that can engage with the screw's head.
The tension cable is often a braided structure made of metal specifically built to withstand cyclical tensile fatigue. The cable may be pre-stretched to minimize further elongation of the cable caused by the application of tension. Turning the rotatable member 20 often supports cable tensions in the range of 5 to 1000 lbs.
Plastic links have a significant problem when used in a surgical theatre, they often cannot be reused due to difficulties in cleaning them. Metallic links, if feasible, would be easier to clean, reducing a costly form of surgical waste.
While there are references in the cited prior art to metal links in a flexible arm linkage assembly dating back to 1990, the inventors have only found plastic links actually in the market. The references in the cited prior art will be discussed in the next few paragraphs.
Prior art, plastic link components were found by the inventors to undergo deflections of up to a factor of 1000% for plastics such as polyethylene when tensioned. Metallic link components typically deflect by less than 50%. This difference in the materials turns out to require an entirely different approach to determining useful metallic links and their contact surfaces. The percentages used above were percent elongation derived from the reference: Materials Science and Engineering, 3rd Edition, W. Callister copyright 1985, which is hereby incorporated by reference.
U.S. Pat. No. 4,949,927 teaches in FIG. 6 and its associated discussion about a link integrating a ball and rod made of aluminum. The inventors found that this link was inoperable, due to a low coefficient of friction. By having the low coefficient of friction, such links slipped easily, far below the point of usefulness.
U.S. Pat. No. 5,899,425 teaches (FIG. 2, Col. 4, lines 7-11) “The flexible, articulating arm 24, as shown in FIG. 2, comprises a series of elements, preferably made of stainless steel . . . . Each element has a convex, spherical surface at one end and a concave, spherical surface at the other end.”
In the Summary of U.S. Pat. No. 5,899,425 (Column 2, lines 35-57), “The bracket is characterized by an interference fit between the spherical balls and their sockets. The diameter of each ball is preferably . . . larger than the diameter of the socket into which it fits. The sockets are hemispherical or almost hemispherical, and their walls are sufficiently flexible to allow the balls to enter them The very small difference in diameter, and the flexibility of the socket walls, allows the balls and sockets to be engaged over an area of contact. The terms ‘area of contact’ and ‘area contact,’ . . . mean contact between a ball and a socket over a substantial area in a common sphere, greater than approximately 20% of the total surface area of the sphere, and is distinguishable from ‘line contact,’ which is contact between a ball and socket over a circular line or a narrow band having an area which is substantially less than 20% of the total area of the sphere corresponding to the larger of the ball or socket. The area of contact extends from the periphery of the socket to the envelope of the perimeter of the cable opening in the concave spherical surface and the circle defining the end of the convex spherical surface adjacent to the cable opening therein. The contact area is preferably approximately 30% to 40% of the total surface area of a corresponding sphere.”
The inventors found that U.S. Pat. No. 5,899,425 was both contradictory and inoperable in its teaching regarding metallic link components. First, maximizing the stainless steel contact area actually reduces the frictional force needed for stiffness. The disclosure from the Summary was appropriate for a plastic link component, but failed to account for the physical characteristics of stainless steel as well as alloys of iron and titanium, which do not deflect anywhere near as much as plastics.
Unlike, the prior art plastic articulating columns that are highly textured and consequently need only low tensile loads for fair rigidity, metallic link contact surfaces behave differently. This is due to the inherently lower interface friction of semi-smooth metallic mating convex and concave surfaces. Friction forces are directly proportional to these distributed contact forces. While two mating spherical surfaces would produce a large contact area, the distributed contact forces are relatively low because they are widely dispersed.
There is an additional problem with highly textured metallic contact surfaces. They would be difficult to clean, posing a health risk if reused in a surgical setting.
Note that a link will also be known herein as a bead.
The inventors know of no disclosure or teaching which provides for an effective metallic link for use in the linkage assembly of a flexible arm. What is needed is such an effective metallic link.
In summary, there is a need for increased stiffness in articulating joints, particularly in flexible arm linkage assemblies. There is a need for reusable links within a surgery, leading to needing metallic, reusable links. And there is a need for reusable links providing increased stiffness in flexible arm linkage assemblies.